Saltmarshes, coastal habitats regularly flooded by tides, trap and store large quantities of organic carbon – known as blue carbon – in their soils. Knowing precisely how much carbon these environments store is critical for conservation and climate action, yet existing methods for measuring this carbon often produce varying results.
Our recent research aimed to find the most reliable method for estimating carbon storage in saltmarsh soils, using two distinctive marshes in Scotland as our study sites – Skinflats in the Firth of Forth and Caerlaverock in the Solway Firth. We compared 22 different methods used globally, ranging from simple calculations to advanced statistical models, to understand how much variation exists among these approaches.
We collected numerous soil samples from both marshes, measuring their carbon content to different depths: one standardised depth of one metre, recommended by international guidelines, and another based on the distinct layers of soil marking the boundary between marsh soils and deeper sediments. Our analysis revealed striking differences: depending on the depth measured and the calculation method, carbon storage estimates varied by as much as 52 times.
Among the methods tested, complex statistical models known as ensemble regression methods proved the most accurate. These techniques combine multiple predictive models, providing detailed maps of how carbon stocks vary across the marsh. Crucially, these methods also highlight areas of uncertainty, helping scientists pinpoint where more data is needed.
In contrast, simpler approaches, such as multiplying average carbon values by marsh area, can offer quick, rough estimates. Still, they risk misrepresenting actual carbon stocks, especially if based on limited samples.
Our findings underscore the importance of choosing the correct sampling depth and calculation method. Using the international standard depth of one metre, while practical, can lead to overestimations if deeper non-marsh sediments are included. Therefore, we advocate for reporting carbon stocks both at this standard depth and for distinct marsh soil layers, providing a clearer picture of true carbon storage.
Detailed carbon maps produced by regression models are particularly valuable for policymakers and conservationists, clearly identifying areas with high carbon stocks and highlighting vulnerable zones. Such maps can guide targeted conservation efforts, ensuring efficient resource allocation.
Ultimately, our research highlights that accurate measurements of saltmarsh carbon storage are essential to support effective climate change mitigation strategies. By understanding the strengths and weaknesses of different measuring techniques, we can ensure that saltmarshes continue to be protected and valued as vital natural carbon reservoirs.